Security checkpoints, such as those located in airports, screen people and packages for contraband, such as weapons or explosives. Various technologies are used at such checkpoints. Typically, individuals pass through metal detection devices. Projection x-ray systems screen baggage and packages. In current conditions of heightened security, passengers can experience long delays in passing through security checkpoints. For baggage, an operator typically reviews all images of screened baggage to determine whether the baggage includes contraband. A typical operator receives extensive training to recognize certain types of objects in an x-ray image. Furthermore, a typical operator receives training to distinguish objects layered within the bags from a single two dimensional x-ray image.
In addition to individuals and carry-on baggage, checked bags are also now scanned at airports. Generally, in the United States, the Transportation Security Administration (TSA) uses computed tomography (CT) scanning for checked bags. CT scanners create a three dimensional image of a bag which allows better differentiation of objects relative to projection x-ray systems. Explosive detection system (EDS) designers specifically developed and deployed CT scanners for the detection of explosives.
As noted above, CT technology is effective for explosive detection. CT machines typically incorporate a rotating ring or “gantry” on which the X-ray source and detectors are mounted. Referring to FIG. 1, a CT scanner 10 typically includes a gantry 12, an x-ray source 14, and a detection device 16. The gantry 12 surrounds a tunnel 18 through which baggage may pass. A conveyor (not shown) can move baggage through the tunnel 18 for scanning. The gantry 12 can rotate about the tunnel 18, and the detection device 16 can collect slices of data. The x-ray source 14 is configured to produce a narrow angle beam 20. The detection device 16 is positioned on the gantry 12 to intersect the x-ray beam 20 passing through the tunnel 18. The detection device 16 comprises multiple x-ray detectors that are typically located at equal distances from the x-ray source 14. The x-ray source 14 and the detection device 16 are sized and positioned so that the entire tunnel 18 falls within the x-ray beam 20. The data from the detection device 16 can be analyzed by a computer to generate a three-dimensional representation of the contents of the bag in the tunnel 18.
Conventional CT scanning and reconstruction used in baggage inspection is slow and cumbersome. Two known methods for CT scanning, i.e., helical and axial. In helical scanning, the detection device includes multiple detection devices disposed adjacent to each other in the direction of travel of the object under inspection, e.g., a bag, and the object is continuously moved through the scanner. The object is moved slowly (but faster than in axial CT) so that a collection of x-ray detectors readings (or interpolation of readings) could be assembled for each rotation which would substantially appear to be from a single plane. The bag is moved approximately the length (up to twice the length) of the multiple detection devices for each rotation. In trans-axial CT scanning, the object under inspection is periodically stopped and a single slice is scanned. The object is then moved a short distance, stopped, and scanned again. Both of these processes result in slow movement of baggage through the scanner. Once the data have been collected, the data are reconstructed to create a three dimensional representation of the baggage. From the three dimensional representation, individual items are reviewed as possible threats. The three dimensional representation, or slices of it, may also be displayed for review by an operator.
The TSA has recognized the need to improve the security process at the passenger checkpoint, such as by using checkpoint EDS. Using conventional EDS at the checkpoint, however, could worsen throughput at already crowded security checkpoints. The TSA and airports are struggling to keep up with passenger loads using today's passenger screening systems and procedures. Lines up to 2 hours can form during peak periods and will likely get worse as TSA headcount is further reduced and passenger loads increase.